The invention relates to the installation and removal of top drive units.
A drilling rig may support use of a top drive unit used to actuate a drill bit via a drill string extending downwardly into a surface of the earth. The top drive unit is typically connected to one end of the drill string with the other end of the drill string connected to the drill bit. As the drill bit bores downwardly into the surface of the earth, a winch of the rig lowers the top drive unit to follow movement of the drill bit and drill string. For purposes of guiding the top drive unit and distributing the torsional forces produced by the top drive unit, the top drive unit is typically slidably mounted to one or two guide rails which extend along an upwardly extending mast of the rig.